Adding one or two storeys to an existing building often looks cheaper than building new. But before adding a floor to a building, you have to prove that the existing structure can carry the extra load, both gravity loads and earthquake forces. This article lists six things to check, from old documents to permits.
1. Old documents: as-built drawings and calculations
Start by collecting everything that exists: as-built drawings, structural working drawings, the calculation report, the soil investigation report and the old building approval (PBG or IMB). These answer the basic questions: what structural system was used, what loads were assumed and what foundation was installed.
In many older buildings the documents are incomplete or no longer match the site because of unrecorded renovations. In that case the engineer measures the members again and, where needed, scans the reinforcement to find the number and position of the bars. Without this data, any calculation is a guess.
2. Actual concrete strength: rebound hammer and cores
The concrete grade on the drawings is not necessarily what is in the building today. Two tests are used most often:
- Rebound hammer test (SNI ASTM C805:2012) measures surface hardness. It is fast, cheap and non-destructive, but it only reflects the surface layer.
- Core drilling (SNI 2492:2018) removes concrete cylinders from structural members and tests their compressive strength in a laboratory. The result is more direct, but the test is semi-destructive, so the number of cores is limited.
Technical material from the Building Materials and Structures Centre of Indonesia's Ministry of Public Works and Housing stresses that non-destructive results such as rebound readings must be correlated with compressive strength, for example with core results, before they are used to judge concrete quality. SNI 2847:2019 likewise allows non-destructive tests to estimate in-place strength only with a valid correlation. A combination of both, not the hammer alone, gives a defensible picture.
Tip: Ask for a test report that marks every test location on the floor plan, so each result can be linked to the member being rechecked.
3. Capacity of columns, beams and slabs
The new floor's load travels down the same path: slab, beams, columns and foundations. The engineer recalculates each member using the actual dimensions, reinforcement and concrete strength. Typical findings include:
- The old roof slab that will become a floor was usually designed for a lighter load than an office or residential floor.
- Ground-floor columns carry the accumulated load of every storey above, so one more storey affects even the lowest columns.
- Existing cracks, corroded bars or sagging reduce capacity. Our article on hairline vs structural cracks explains how to read crack patterns.
4. Foundations and soil
Foundations often decide whether an extra storey is feasible. Piles designed for a certain load do not necessarily have spare capacity. Old soil data must be reviewed, and if it is missing or too limited, a new soil investigation may be needed. See CPT vs SPT for the difference between the two common tests.
If the foundations fall short, strengthening them is usually harder than strengthening columns, because the work happens under an occupied building with little headroom.
5. Earthquake forces and the current code
An extra storey adds mass, and more mass means larger earthquake forces. Many older buildings were designed to an earlier edition of the seismic code, while the current one is SNI 1726:2019. The assessment must check whether the existing lateral system still complies, taking into account the building's use and risk category, as explained in seismic risk categories in SNI 1726.
For existing buildings, assessment practice in Indonesia also refers to methods such as ASCE 41, which splits the evaluation into tiers of increasing detail. The outcome may be: the structure complies, it needs local strengthening, or the extra storey is not advisable.
6. Permits: amended PBG and SLF
Adding a storey is not only a technical matter. Government Regulation (PP) 16/2021 defines the building approval (PBG) as approval to build, alter, extend, reduce and/or maintain a building (Article 1 point 17). Article 253 requires the PBG to be obtained before construction, and Article 262(2) includes a change in floor area within the scope of an amended PBG. Structural documents, including calculations, form part of that application. Once the work is finished, the building also needs an SLF that reflects its new condition.
Adding a floor to a building: findings and next steps
| Assessment finding | Likely next step |
|---|---|
| All members and foundations are adequate | Design the new floor and apply for an amended PBG |
| Some columns or beams fall short | Local strengthening before adding the floor |
| Foundations lack capacity | Study foundation strengthening or limit the number or weight of new floors |
| Lateral system does not comply | Global strengthening or rethink the plan |
Ways to reduce weight, such as a steel structure or lighter floor materials, still have to be calculated, not assumed to be safe.
Conclusion
Adding a floor is possible, but only once the six points above are answered with data: old documents, actual concrete strength, member capacity, foundations, earthquake loads and permits. Every building is different, so the final decision must rest on calculations by a competent structural engineer, not on experience from another building.
Want to know whether your building can take an extra storey? Start with a structural audit by Stadin.
This article provides general information. Every project still needs to be analysed and calculated by a competent structural or geotechnical engineer, in line with the applicable SNI codes and regulations.
